25.2 On some Effects of a Chemically Clean Surface. 



has been noticed in other arts. In an old French treatise on 

 enamel-printing {Diet, des Arts et Metiers, Lyons, 1801) the 

 artist is cautioned not to let any one approach his work who has 

 been eating garlic or taking mercurial medicines." 



Variations in the amount of adhesive force, and distinctions be- 

 tween chemically clean and unclean surfaces, are constantly 

 being met with on the great scale of nature. Water does not 

 adhere to an oily or greasy surface; but air does so abundantly; 

 and it seems necessary that it should do so in order to carry on 

 the fermenting processes by which offensive organic matter is 

 got rid of. For an opposite reason, salt will adhere to fatty 

 matter, and so prevent those chemical changes which produce 

 fermentation. It is seldom that we get perfect adhesion be- 

 tween water and a natural organic solid ; mere exposure to the 

 air prevents it more or less from retaining a chemically clean 

 surface, even if it had one on its first appearance, as in the case 

 of the young leaf or blade of grass. If the surface were chemi- 

 cally clean, the rain and the dew would completely wet it, eva- 

 poration would be more difficult, and there would be danger, 

 from frequent soaking, of organic injury and even decay. But, 

 in consequence of imperfect adhesion, and the presence also of a 

 downy, powdery, oily, varnished, or rough irregular structure in 

 foliage and flowers, and in the natural coverings of animals, the 

 moisture forms in globules, which refresh the plant &c. while 

 they remain, but are quickly dispersed by the wind or the heat 

 of the sun. Inorganic matter, on the contrary, is rendered 

 more or less chemically clean by every shower, and is more or 

 less completely wetted; so that the roots of plants, or those 

 parts which require most moisture, obtain it by a simple varia- 

 tion in the adhesive force of the constituents of the soil for water 

 as compared with the living structures that grow out of it. The 

 roots also are more readily wetted than the foliage or parts ex- 

 posed to the air. 



It is highly probable that all healthy animal and vegetable se- 

 cretions are chemically clean, as well as the surfaces that secrete 

 them. I have found that camphor will spin upon two or three 

 such fluids when perfectly fresh ; but they almost immediately 

 change by exposure to the air, and then the camphor fragments 

 are inactive. 



There are various modes of obtaining chemically clean sur- 

 faces in flasks and other apparatus. Washing out with strong 

 sulphuric acid, or with a strong solution of caustic potash, or 

 with spirits of wine, and then rinsing with water, is generally 

 sufficient. Soda-water poured into a test-glass thus prepared 

 will not part with a single bubble of gas to the sides. When 

 flasks are cleaned for highly saturated solutions of salts which 



